It's the vertical support bracket that attaches the upper cross bar, stone deflector and bottom airshield. The only number I see is stamped into this piece is SS03901.
*UPDATE*
The "SS03901" number is not pulling up anything. When I search "lock vertical support" I get some results but only for years up to 1965 that from 1970 and up so I'm kind of at a loss because all of those options are very different from the one i need.
The one stamped on mine is "SS05001" which didn't pull up any helpful results...I'll look into the number you have provided and update.